Sputter coating refers to a process for coating a substrate, such as a semiconductor wafer, within a vacuum processing chamber of a sputtering system. In the sputter coating process, an applied electric field positively biases the wafer relative to an oppositely mounted, negatively biased target made of sputtering material. Once the processing chamber is evacuated, an inert gas is introduced into the chamber at a low pressure, and the applied electric field ionizes the process gas. As a result, positive ions from the gas bombard the target to cause sputtering of the target material onto the wafer in a thin film. A magnet or electromagnet may be located behind the target to provide a magnetic field above the surface of the target facing the wafer to confine the ion "plasma" adjacent the target, and thereby enhance the sputter coating operation.
Sputtering targets are typically formed as a generally circular disk of target material, such as aluminum alloys, gold, silver, copper, titanium, titanium-tungsten or platinum. The disk of target material may be soldered or otherwise bonded to a supporting target backplate to form a replaceable sputtering target assembly. During the sputtering operation, material is sputtered from the top surface of the target and deposited on the wafer. The sputtering material typically erodes unevenly across the width or face of the target exposed to the wafer, with some areas of the target eroding more quickly than other areas.
To overcome this problem, some sputtering equipment employs a variation of the magnetic field, or multiple, non-planar erosion zones on the target, or both, to create a generally uniform sputtering rate across the face of the target. Typically, the outer radial region of the target has been made thicker than the central region of the target. The target may include a concave region formed in the target face or even a hole formed through the center of the target. Sputtering material is eroded from the target until the target is no longer able to provide the desired coating features on the wafer. At that time, the target assembly, consisting of the eroded target and backplate, is replaced by a new target assembly.
In titanium nitride (TiN) sputtering, for example, a titanium sputtering target and backplate assembly is mounted in the vacuum processing chamber with exposed surface of the target facing a wafer. The vacuum chamber is evacuated, and then filled with nitrogen gas that ionizes in the presence of the applied electric field. Positive ions from the plasma process gas bombard the top surface of the target and cause titanium particles to be sputtered toward the wafer. During the sputtering process, the titanium particles chemically react with the nitrogen process gas to form a thin film of titanium nitride on the wafer surface.
An important aspect in sputter coating of wafers is the purity of the film deposited onto the wafer. As the amount of contaminants within the sputter processing chamber increases, the wafer product yield decreases as impurities are formed on the wafer. For example, in titanium nitride sputtering, TiN nodules are known to form on the sputtering face of the target as material from the central portion of the target is sputtered and redeposited on the outer peripheral edge of the target face rather than on the wafer. During the sputtering operation, the TiN nodules have a tendency to flake and generate contaminating particles that adversely affect the purity of the deposited titanium nitride film on the wafer. Since TiN particle generation becomes worse with increasing use of the target, the target must be periodically conditioned to maintain an acceptable device yield.
Target conditioning is achieved by conducting titanium-only sputtering on non-product substrates. The titanium-only sputtering causes the TiN nodules held on the target to be released and deposited on the non-product substrates. While the periodic conditioning prolongs the usable life of the target by removing the contaminating TiN nodules, it requires stoppage of the wafer coating production line and results in downtime of the sputtering system. Thus, notwithstanding known contouring of sputtering targets and modifications to operation of sputtering systems, nodule formation on targets has been a problem, particularly in titanium nitride sputter deposition.
In the past, one-piece sputtering targets have been made that include steep-angled bevels formed adjacent the outer edge of the target to reduce redeposition of TiN particles near the outer surface of the target. In U.S. Pat. No. 5,538,603, for example, bevels are formed adjacent the outer edge of the target that taper at an angle of at least 30.degree. with respect to the planar face of the target, and preferably at a greater angle, i.e., 70.degree. , such that the trajectory of the backscattered atoms will cause the atoms to miss the outer surface of the target altogether. The steep angles of the bevels are also chosen to reduce the thickness of the target adjacent its outer peripheral edge to increase the sputter deposition rate near the tapered target edge. Thus, backscattered TiN particles that collide and redeposit on the target are more likely to be resputtered onto the wafer or substrate. However, the steep tapered targets are single-piece targets that do not encounter the same TiN nodule formation problems associated with sputtering targets mounted to a target backplate.
Accordingly, there is a need for a sputtering target and backplate assembly that reduces generation of contaminating particles from nodules that may form during a sputtering operation. There is also a need for a sputtering target and backplate assembly that requires less periodic conditioning during the life of the target, thereby resulting in less system downtime and a reduction in preventative maintenance.